The present invention is directed to a novel copolymer which can be utilized as a cement admixture to greatly increase the fluidity or slump characteristics of hydraulic cement compositions without causing excessive retardation or prevent set of the treated composition.
Hydraulic cement compositions are brought into a workable form by mixing the solid components with an amount of water which is greater than that required to hydrate the cement components therein. The mixed mineral binder composition is poured into a form and allowed to harden at atmospheric temperature. During the hardening, some of the excess water remains, leaving cavities in the formed structural unit and, thus, reduces the mechanical strength of the resultant unit. It is well known that the compressive strength of the resultant structure generally bears an inverse relationship to the water-cement ratio of the starting mix. The desire to use smaller quantities of water is limited by the required flow and workability properties of the fresh mixture.
It is desired in many applications to use unset cement compositions which are of low viscosity or even self-leveling and, at the same time, are capable of forming a set cement of high compressive strength (via low water-cement ratio). In structural cement compositions, for example, it is highly desirable to maintain very low water content in order to achieve high strength in the final product. On the other hand, it is desirable that the unset composition have fluid properties to enhance mixing to cause uniform distribution of the liquid (water) in the solid components, pumpability to permit the unset cement composition to be carried to the needed structural site and flowability to permit the unset cement composition to be readily shaped into the desired form.
Cement admixtures (The term "admixture" as used herein and in the appended claims is a term of art referring to compounds and compositions added to cement mixtures or compositions to alter their properties. The term does not imply that the components of an admixture do or do not interact to cause the desired result) capable of causing the above-described viscosity reducing characteristics are known. These materials are generally categorized as "water-reducing agents" if they are capable of modifying viscosity to a limited degree or as "high range water-reducing agents" or "superplasticizers" if they have the ability to permit large water cuts in the cement mixture while maintaining fluidity or cause large increases in fluidity at constant water content. Lignin sulfonates and polysaccharides are known water reducing agents while sulfite-modified condensation products of melamine-formaldehyde or sulfonate-modified condensation products of naphthalene-formaldehyde are commercially available superplasticizers. While these admixtures have the advantage of increasing initial fluidity, they also have the disadvantage of increasing the rate at which the cement composition loses its fluidity or slump. H. P. Preiss and H. R. Sasse, in Superplasticizers in Concrete, Vol. II, Ed. by V. M. Malhotra et al, pages 733-750, compare the effects of various known water-reducing agents and superplasticizers including sulfonated melamine-formaldehyde condensates, sulfonated naphthalene-formaldehyde condensates, lignin-sulfonates and polystyrene sulfonates. The study concludes that very high dosages of any of the studied admixtures are needed to appreciably increase the flow of cement compositions, yet such dosages enhance the rate of slump loss and tend to retard or prevent set.
In addition to the above mentioned known polymeric materials, various polyacrylates have been considered as cement superplasticizer admixtures. Polyacrylates of high molecular weight have been found to be unsuitable as a superplasticizer as they cause flocculation of cement slurries. More recently, certain acrylic acid-hydroxyalkyl acrylate copolymers have been suggested as a flow enhancing agent. These copolymers are required to be of low molecular weight and have a high acrylic acid content to impart water solubility to the polymeric composition and to have the copolymer exhibit stability in a cementitious environment. Although these copolymers do plasticize cement compositions, they impart excessive set retardation to the composition so that they are not suitable for general application.
It is highly desired to have superplasticizer admixture compositions which can impart a high degree of fluidity to cement compositions and can achieve this result without adverse effect of set retardation.